1. Field of the Invention
This invention relates to a process for the preparation of Mgxe2x80x94Al anionic clays.
2. Prior Art
Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein hydroxyl is the predominant anion present.
In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers may contain anions such as NO3xe2x88x92, OH, Clxe2x88x92, Br31, I31, SO42xe2x88x92, SiO32xe2x88x92, CrO42xe2x88x92, BO32xe2x88x92, MnO4xe2x88x92, HGaO32xe2x88x92, HVO42xe2x88x92, ClO431, BO32xe2x88x92, pillaring anions such as V10O28xe2x88x926 and MO7O246xe2x88x92, monocarboxylates such as acetate, dicarboxylates such as oxalate, and alkyl sulfonates such as laurylsulfonate.
It should be noted that a variety of terms are used to describe the material that is referred to in this specification as an anionic clay. Hydrotalcite-like and layered double hydroxide is interchangeably used by those skilled in the art. In this specification we refer to these materials as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials. The anionic clays referred to in this document are anionic clays having the conventional 3R1 stacking. These clays have regular well-formed layers of platelets that are arranged in the bookstack form. A more detailed description of this and other types of anionic clays can be found in the publications in Clay and Clay Minerals, Vol. 41, No. 5, pp. 551-557 and pp. 558-564 of Bookin and Drits.
The preparation of anionic clays has been described in many prior art publications. Two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarised: F. Cavani et al xe2x80x9cHydrotalcite-type anionic clays: Preparation, Properties and Applications,xe2x80x9d Catalysis Todayxe2x80x9d, 11 (1991) Elsevier Science Publishers B. V. Amsterdam; and J P Besse and others xe2x80x9cAnionic clays: trends in pillary chemistry, its synthesis and microporous solidsxe2x80x9d (1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N.Y.
In these reviews the authors state that a characteristic of anionic clays is that mild calcination at 500xc2x0 C. results in the formation of a disordered MgO-like product. Said disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. In this specification we refer to said disordered MgO-like materials as Mgxe2x80x94Al solid solutions. Furthermore, these Mgxe2x80x94Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure.
Two types of anionic clay preparation are described in these reviews. The most conventional method is co-precipitation (in Besse this method is called the salt-base method) of a soluble divalent metal salt and a soluble trivalent metal salt, optionally followed by hydrothermal treatment or aging to increase the crystallite size. The second method is the salt-oxide method in which a divalent metal oxide is reacted at atmospheric pressure with a soluble trivalent metal salt, followed by aging under atmospheric pressure. This method has only been described for the use of ZnO and CuO in combination with soluble trivalent metal salts.
For work on anionic clays, reference is further made to the following articles:
Helv. Chim. Acta, 25, 106-137 and 555-569 (1942)
J. Am. Ceram. Soc., 42, no. 3,121 (1959)
Chemistry Letters (Japan), 843 (1973)
Clays and Clay Minerals, 23, 369 (1975)
Clays and Clay Minerals, 28, 50 (1980)
Clays and Clay Minerals, 34, 507 (1996)
Materials Chemistry and Physics, 14, 569 (1986).
In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation.
Several patent applications relating to the production of anionic clays from inexpensive raw materials have been published. These materials include magnesium oxide and aluminum trihydrate.
WO 99/41198 relates to the production of anionic clay from two types of aluminum compounds and a magnesium source. One of the aluminum sources is aluminum trihydrate or a thermally treated form thereof.
WO 99/41196 discloses the preparation of anionic clays with acetate as the charge balancing anion from magnesium acetate, another magnesium source and aluminum trihydrate.
In WO 99/41195 a continuous process is described for the production of a Mgxe2x80x94Al anionic clay from a Mg source and aluminum trihydrate.
WO 99/41197 discloses the production of an anionic clay-containing composition comprising a Mgxe2x80x94Al anionic clay and unreacted aluminum trihydrate or a thermally treated form thereof. Milling of the magnesium source is not mentioned in this document.
Several patents in the name of Alcoa describe the synthesis of hydrotalcites, i.e. anionic clays, out of magnesium oxide and a transition alumina, in a batch-wise manner and under non-hydrothermal conditions: U.S. Pat. Nos. 5,728,364, 5,728,365, 5,728,366, 5,730,951, 5,776,424 and 5,578,286. Comparative Examples 1-3 presented in these patents indicate that upon using aluminum trihydrate as aluminum source, anionic clays are not formed.
There are many applications of anionic clays. These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular Van Broekhoven (U.S. Pat. Nos. 4,956,581 and 4,952,382) has described their use in SOx abatement chemistry.
In one embodiment, this invention relates to a process for preparing a 3R1-type crystalline anionic clay comprising the steps of:
a) preparing an aqueous precursor mixture comprising aluminum trihydrate or a thermally treated form thereof and a magnesium source, the magnesium source being milled before use and/or when present in the precursor mixture,
b) aging the precursor mixture at a temperature in the range of 30xc2x0-100xc2x0 C. to obtain the crystalline clay product, and
c) optionally shaping the product of b).
Other embodiments of the invention relate to precursor mixture composition, process conditions and additional process steps.